Alzheimer's disease (AD) is a progressive disease known generally as senile dementia. Broadly speaking the disease falls into two categories, namely late onset and early onset. Late onset, which occurs in old age (65+ years), may be caused by the natural atrophy of the brain occurring at a faster rate and to a more severe degree than normal. Early onset AD is much more infrequent but shows a pathologically identical dementia with brain atrophy which develops well before the senile period, i.e., between the ages of 35 and 60 years.
Alzheimer's disease is characterized by the presence of numerous amyloid plaques and neurofibrillary tangles (highly insoluble protein aggregates) present in the brains of AD patients, particularly in those regions involved with memory and cognition. While in the past there was significant scientific debate over whether the plaques and tangles are a cause or are merely the result of AD, recent discoveries indicate that amyloid plaque is a causative precursor or factor. In particular, it has been discovered that the production of .beta.-amyloid peptide, a major constituent of the amyloid plaque, can result from mutations in the gene encoding amyloid precursor protein, a protein which when normally processed will not produce the .beta.-amyloid peptide. It is presently believed that a normal (non-pathogenic) processing of the .beta.-amyloid precursor protein occurs via cleavage by a putative ".alpha.-secretase" which cleaves between amino acids 16 and 17 of the protein. It is further believed that pathogenic processing occurs via a putative ".beta.-secretase" at the amino-terminus of the .beta.-amyloid peptide within the precursor protein. Moreover, .beta.-amyloid peptide appears to be toxic to brain neurons, and neuronal cell death is associated with the disease.
.beta.-amyloid peptide (also referred to as A4, .beta.AP, A.beta., or A.beta.P; see, U.S. Pat. No. 4,666,829 and Glenner and Wong (1984) Biochem. Biophys. Res. Commun. 120: 1131) is derived from .beta.-amyloid precursor protein (.beta.APP), which is expressed in differently spliced forms of 695, 751, and 770 amino acids. See, Kang et al. (1987) Nature 325: 773; Ponte et al. (1988) Nature 331: 525; and Kitaguchi et al. (1988) Nature 331: 530. Normal processing of amyloid precursor protein involves proteolytic cleavage at a site between residues Lys.sup.16 and Leu.sup.17 (as numbered for the .nu.AP region where Asp.sup.597 is residue 1 in Kang et al. (1987)), supra, near the transmembrane domain, resulting in the constitutive secretion of an extracellular domain which retains the remaining portion of the .beta.-amyloid peptide sequence (Esch et al. (1990) Science 248:1122-1124). This pathway appears to be widely conserved among species and present in many cell types. See, Weidemann et al. (1989) Cell 57:115-126 and Oltersdorf et al. (1990) J. Biol. Chem. 265:4492-4497. This normal pathway cleaves within the region of the precursor protein which corresponds to the .beta.-amyloid peptide, thus apparently precluding its formation. Another constitutively secreted form of .beta.APP has been noted (Robakis et al. Soc. Neurosci. Oct. 26, 1993, Abstract No. 15.4, Anaheim, Calif.) which contains more of the .beta. AP sequence carboxy terminal to that form described by Esch et al. supra.
Golde et al. (1992) Science 255:728-730, prepared a series of deletion mutants of amyloid precursor protein and observed a single cleavage site within the .beta.-amyloid peptide region. Based on this observation, it was postulated that .beta.-amyloid peptide formation does not involve a secretory pathway. Estus et al. (1992) Science 255:726-728, teaches that the two largest carboxy terminal proteolytic fragments of amyloid precursor protein found in brain cells contain the entire .beta.-amyloid peptide region.
Recent reports show that soluble .beta.-amyloid peptide is produced by healthy cells into culture media (Haass et al. (1992) Nature 359:322-325) and in human and animal CSF (Seubert et al. (1992) Nature 359:325-327). Palmert et al. (1989) Biochm. Biophys. Res. Comm. 165:182-188, describes three possible cleavage mechanisms for .beta.APP and presents evidence that .beta.APP cleavage does not occur at methionine.sup.596 in the production of soluble derivatives of .beta.APP. U.S. Pat. No. 5,200,339, discusses the existence of certain proteolytic factor(s) which are putatively capable of cleaving .beta.APP at a site near the .beta.APP amino-terminus.
The APP gene is known to be located on human chromosome 21. A locus segregating with familial Alzheimer's disease has been mapped to chromosome 21 (St. George Hyslop et al (1987) Science 235: 885) close to the APP gene. Recombinants between he APP gene and the AD locus have been previously reported (Schellenberg et al. (1988) Science 241: 1507; Schellenberg et al. (1991) Am. J. Hum. Genetics 48: 563; Schellenberg et al. (1991) Am. J. Hum. Genetics 49: 511, incorporated herein by reference).
The identification of mutations in the amyloid precursor protein gene which cause familial, early onset Alzheimer's disease is evidence that amyloid metabolism is the central event in the pathogenic process underlying the disease. Four reported disease-causing mutations include with respect to the 770 isoform, valine.sup.717 to isoleucine (Goate et al. (1991) Nature 349: 704), valine.sup.717 to glycine (Chartier Harlan et al. (1991) Nature 353: 844), valine.sup.7 l.sup.7 to phenylalanine (Murrell et al. (1991) Science 254: 97) and with respect to the 695 isoform, a double mutation changing lysine.sup.595 -methionine.sup.596 to asparagine.sup.596 -leucine.sup.596 (Mullan et al. (1992) Nature Genet 1: 345; Citron et al. (1992) Nature 360: 672) referred to as the Swedish mutation. APP alleles which are positively correlated with AD are termed "disease-associated alleles".
The development of experimental models of Alzheimer's disease that can be used to define further the underlying biochemical events involved in AD pathogenesis would be highly desirable. Such models could presumably be employed, in one application, to screen for agents that alter the degenerative course of Alzheimer's disease. For example, a model system of Alzheimer's disease could be used to screen for environmental factors that induce or accelerate the pathogenesis of AD. In contradistinction, an experimental model could be used to screen for agents that inhibit, prevent, or reverse the progression of AD. Presumably, such models could be employed to develop pharmaceuticals that are effective in preventing, arresting, or reversing AD.
Unfortunately, only humans and aged non-human primates develop any of the pathological features of AD; the expense and difficulty of using primates and the length of time required for developing the AD pathology makes extensive research on such animals prohibitive. Rodents do not develop AD, even at an extreme age. It has been reported that the injection of .beta.-amyloid protein (.beta.AP) or cytotoxic .beta.AP fragments into rodent brain results in cell loss and induces an antigenic marker for neurofibrillary tangle components (Kowall et al. (1991) Proc. Natl. Acad. Sci. (U.S.A.) 88: 7247). Mice which carry an extra copy of the APP gene as a result of partial trisomy of chromosome 16 die before birth (Coyle et al. (1988). Trends in Neurosci. 11: 390). Since the cloning of the APP gene, there have been several attempts to produce a mouse model for AD using transgenes that include all or part of the APP gene, unfortunately much of the work remains unpublished since the mice were nonviable or failed to show AD-like pathology. At least two published reports were retracted because of irregularities in reported results (Marx J Science 255: 1200; Wirak et al. (1991) Science 253: 323; Kawabata et al. (1991) Nature 354: 476; Kawabata et al. Nature 356: 23; Quon et al. (1991) Nature 352: 239; Marx Science 259: 457).
Thus, there is a need in the art for transgenic nonhuman animals harboring an intact disease-associated APP gene, either a human disease-associated allele such as a polynucleotide encoding a human APP protein comprising the Swedish mutation, or a complete genomic copy (or minigene) of the Swedish mutation APP gene.
Alternatively, a mutated rodent (e.g., murine) allele which comprises sequence modifications which correspond to a human APP sequence comprising the Swedish mutation can be substituted. Cell strains and cell lines (e.g., astroglial cells) derived from such transgenic animals would also find wide application in the art as experimental models for developing AD therapeutics and as a convenient source of APP protein comprising the Swedish mutation. Moreover, transgenic non-human animals comprising a transgene encoding a Swedish mutation APP protein and lacking functional endogenous APP gene loci (i.e., having an APP "knockout" background) would be a convenient source of Swedish mutation APP protein in a backgound lacking other APP proteins that do not comprise the Swedish mutation.
Based on the foregoing, it is clear that a need exists for nonhuman cells and nonhuman animals harboring one or more transgenes encoding an APP gene comprising the Swedish mutation. Thus, it is an object of the invention herein to provide methods and compositions for transferring transgenes and homologous recombination constructs into mammalian cells, especially into embryonic stem cells. It is also an object of the invention to provide transgenic nonhuman cells and transgenic nonhuman animals harboring one or more Swedish mutation APP transgenes of the invention. Of further interest to the present invention are the application of such transgenic animals as in vivo systems for screening candidate drugs for the ability to inhibit or prevent the production of pathogenic .beta.-amyloid plaque. It would be desirable to provide methods and systems for screening test compounds for the ability to inhibit or prevent the conversion of amyloid precursor protein to pathogenic .beta.-amyloid peptide. In particular, it would be desirable to base such methods and systems on metabolic pathways which have been found to be involved in such conversion, where the test compound would be able to interrupt or interfere with the metabolic pathway which leads to conversion. Such methods and transgenic animals should provide rapid, economical, and suitable for screening large numbers of test compounds.
The references discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention.